Compositions for coating industrial components and assemblies are becoming increasingly important. For example, many mechanical parts and fasteners are coated with a composition to improve the aesthetics and overall appearance of the part or fastener, particularly when the part is visible in the final assembled product. Additionally, mechanical fasteners such as bolts or screws may be colored to simplify assembly or disassembly of a manufactured product. These compositions often contain pigments or other coloring agents as desired, to impart a certain color or appearance to the coated part.
Many mechanical components used in automobiles are coated with a darkening paint or composition to impart a black, gray, or dark finish. Since many mechanical components must, as a result of strength requirements, be metal; without such coatings, the metallic components are silver or at least shiny in appearance. In order to impart a black or dark appearance to such components, it is necessary to apply a suitable coating.
Various compositions are known for imparting a black or dark color to a metallic part. Many of these compositions are commercially available. However, for many applications, in order to effectively cover the silver and shiny metallic surface of the part, multiple coats of the coloring coating must be applied. This is undesirable because such compositions are often relatively expensive. And, multiple coating operations are labor intensive. Accordingly, there is a need for a technique to reduce the expense otherwise associated with the use of these coloring coatings.
In addition to applying a composition to color a metallic part, other compositions are often applied to the part to impart other physical characteristics. Corrosion resistance is a desirable property for metallic parts, and particularly for such parts used in automotive applications. The art is replete with a wide variety of compositions for imparting corrosion-resistant properties to a metal surface. Coating compositions have evolved along with the changing technology of alloys and understanding of the science of corrosion.
A factor affecting the evolution of corrosion-inhibiting compositions is the relative toxicity or environmental impact of the composition or its components. For this reason, molybdate has been investigated as a suitable anti-corrosion agent, and particularly as a replacement for toxic chromium or chromium-based compounds.
Molybdenum and compounds thereof have long been recognized as corrosion inhibitors. For example, U.S. Pat. No. 4,409,121, herein incorporated by reference, describes corrosion inhibiting compositions containing a molybdate salt. In the background section of that patent, the '121 patent notes other patents directed to corrosion inhibiting compositions containing molybdate such as U.S. Pat. Nos. 4,176,059 and 4,217,216; both of which are hereby incorporated by reference.
Similarly, U.S. Pat. No. 4,440,721, herein incorporated by reference, describes compositions for inhibiting mineral scale and corrosion in the presence of aqueous liquids. The compositions of the '721 patent include one or more water-soluble molybdate compounds. Other patents directed to aqueous compositions containing molybdenum compounds include U.S. Pat. Nos. 3,030,308; 2,147,409; and 2,147,395; all of which are hereby incorporated by reference. These compositions are however, generally directed to anti-freeze compositions.
Further investigation into the corrosion inhibiting properties of molybdate led to U.S. Pat. No. 4,548,787, herein incorporated by reference. The '787 patent describes a composition that protects against cavitation-erosion and corrosion of aluminum in aqueous liquids. That composition is based upon the combination of a phosphate and certain water-soluble agents which may include a water-soluble molybdate compound.
Additional mention was made of the use of water-soluble salts of molybdenum in corrosion inhibiting mixtures based on a particular class of polymers, in U.S. Pat. No. 4,640,793, herein incorporated by reference.
Perhaps the most relevant prior work in the patent literature is U.S. Pat. No. 4,798,683, herein incorporated by reference. The '683 patent is directed to methods of controlling corrosion by the use of molybdate compositions. Specifically, the '683 patent describes methods and compositions for inhibiting the corrosion of metallic surfaces in contact with aqueous systems. The compositions of the '683 patent contain a molybdate ion source and certain water-soluble components. The '683 patent discloses molybdate ion sources as including magnesium molybdate, ammonium molybdate, lithium molybdate, sodium molybdate, and potassium molybdate.
Another interesting, although less relevant, prior work involving molybdate compositions is by Philippe Lienard and Clement Pacque entitled, “Analysis of the Mechanism of Selective Coloration Facilitating the Identification of Various Phases in Aluminum-Silicon-Copper Casting Alloys,” Homes Et Fonderie, June-July 1982, p. 27-35. In that paper, an aqueous composition of 0.5 weight percent ammonium heptamolybdate and 3 weight percent ammonium chloride was used to emphasize and highlight grain boundaries in various alloys that were the subject of their work. There was no attempt to impart corrosion inhibiting properties to the alloys under review by the aqueous molybdate composition.
Although satisfactory in many respects, much of the prior art is directed to applications involving corrosion control in heat transfer systems and not to coating compositions for corrosion control. The two applications have significantly different criteria. Additionally, many of the prior art anti-corrosion compositions contain numerous other agents, many of which are exotic, costly, or highly toxic. Accordingly, there remains a need for a composition and method for readily imparting corrosion resistance to a metal surface. Moreover, prior art anti-corrosion compositions do not address the concerns over improving the aesthetics of metallic parts and fasteners, and particularly imparting a black or dark color to the coated part. The previously noted work by Lienard and Pacque was not directed to providing a dark surface to a metal. Moreover, Lienard and Pacque never described any aspect concerning a corrosion inhibiting composition for their alloy. Instead, they used the noted molybdate composition to render grain boundaries of an aluminum-silicon-copper alloy more visible, i.e. to increase the contrast between certain regions of a metal surface. Accordingly, it would be desirable to provide a composition and method for readily darkening a metallic surface. Moreover, it would be desirable to provide a composition and technique for reducing the contrast between a shiny or silvery metal surface and a dark pigmented or colored top coat. Furthermore, it would be particularly desirable to provide a composition and method for simultaneously imparting anti-corrosion, or at least corrosion-resistant properties and darkening the outer surface of a metal part.